Dark lake surrounded by military group (5)
I believe the answer is:
unlit
'dark' is the definition.
(similar in meaning)
'lake surrounded by military group' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'surrounded by' indicates putting letters inside.
'military group' becomes 'unit' (group of soldiers).
'l' placed within 'unit' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "Without illumination" , "Until (anag.)" , "such as dark alley?" , "Shady" , "dim" .)